Reactive atomized zero valent iron enriched with sulfur and carbon to enhance corrosivity and reactivity of the iron and provide desirable reduction products

ABSTRACT

Iron, in the form of particles or iron wool, is used for the remediation of contaminated water. For ensuring that the process generally follows preferred chemical pathways resulting in non-toxic end products, and for providing greater rates of contaminant reduction, the iron is enriched with graphite carbon, at least 4% by weight, and sulfur, at least 0.5% by weight.

BACKGROUND OF THE INVENTION

This invention relates to the use of enriched atomized Zero Valent Iron,typically in the form of particles, for remediation of contaminatedwater, and particularly to means for controlling the contaminantreduction pathways, which are followed during the remediation process.

It is known to use enriched atomized Zero Valent Iron (ZVI) particles toachieve rapid reduction rates and desirable byproducts for the treatmentof large volumes of ground and surface waters contaminated with organicand inorganic pollutants. Nano-scale and micro-scale ZVI particles havebeen used for insitu and surface treatment of contaminated water, but ameans to control contaminant reduction pathways to yield rapid reductionrates and desirable reduction end products has generally not beenavailable.

The treatment of soluble inorganic and organic pollutants using ZVIparticles requires the presence of water, which corrodes the ZVI to formelectrons and ferrous ions. The corrosion of the ZVI particles iscarried out in the presence of dissolved oxygen, which can enhance thecorrosion rate according to accepted iron corrosion chemistry.

The reduction of dissolved phase pollutants with ZVI can occur throughtwo mechanisms: 1) direct electron transfer from the corroding iron tothe dissolved contaminant compound or 2) indirect reduction by atomichydrogen formed from the catalytic reduction of water by electrons atcatalytic sites that exist on the surface of the ZVI. Direct electrontransfer primarily favors the sequential reduction pathways(hydrogenolysis), which generally lead to the formation of undesirablereaction products. For example, direct electron reduction ofTrichloroethylene (TCE) first forms 1,2,cis Dichloroethylene (DCE) andthen Vinyl Chloride (VC), daughter products both of which are consideredmore toxic than TCE.

Conversely, the indirect reduction of pollutants involving atomichydrogen appears to follow elimination and hydrogenation reductionpathways that bypass the hydrogenolysis pathway to yield the moredesirable ethylene and ethane. However, as above noted, in any givenremediation process, it has not been always possible to control whichremediation mechanism is actually followed.

SUMMARY OF THE INVENTION

For better ensuring that a given ZVI contaminant reduction process favorreactions that follow elimination and hydrogenation pathways, as well aspossibly providing greater rates of contaminate reduction, the ZVI,typically in the form of particles, but also in the form of an ironwool, is enriched with at least 4% by weight of graphite carbon and 0.5%by weight of sulfur.

DETAILED DESCRIPTION

It has been reported that the percent loadings of ZVI by weight relativeto 1) the weight of volume of contaminated water or 2) weight of volumeof contaminated geological formation being treated influences thereduction pathways for less reactive pollutants. The use of low loadingsof 0.16% by weight of electrolytic type ZVI particles for the reductionof perchloroethylene (PCE), TCE and VC favor elimination andhydrogenation reduction pathways by atomic hydrogen rather than directelectron reduction that favors the hydrogenolysis reduction pathway.

The elimination reaction pathway observed for the reduction of PCE, TCE,and VC for a 0.16% ZVI loading was reported to represent 87%, 97% and94% of the reactions pathways involved. As an example, the results ofthe analysis for the distribution of reaction products from thereduction of TCE show the DCE (hydrogenolysis pathway) to be 1.7, mole%, the ethylene to be 77 μmole % (elimination pathway) and the ethane tobe 21, mole % (hydrogenation pathway). The 1^(st) order rate constantfor the observed reduction of TCE was reported to be only 0.0020 hr⁻¹[Arnold, W. et al “Pathways and Kinetics of Chlorinated Ethylene andAcetylene Reaction with Fe(0) Particles” Env. Sci. Tech 2000, 34(1794-1806)]. We have found that the first order constant for thereduction of TCE can be increased by raising the electrolytic type ZVIto a loading of 5.3% by weight. The observed rate constant wasdetermined to be increased eight times to 0.0163 hr⁻¹. However; at theseloadings the hydrogenolysis reaction pathways has a greater influence onthe distribution of reaction products by forming more toxic reactionproducts. The cis DCE is increased from 1.7 μmole % to 24 μmole %, theethylene is decreased from 77 μmole % to 18 μmole % and the ethaneincreases slightly from 21 μmole % to 24 μmole %. Apparently, at theseloadings the catalytic sites on the electrolytic type ZVI are becomingsaturated with electrons, which appear to inhibit atomic hydrogenformations. As a result, greater amounts of electrons are available fordirect reduction of the TCE by hydrogenolysis to form cis DCE.

The results of corrosion studies employing electrolytic type ZVIparticles at higher loadings than 0.16% by weight show apparent hydrogengas production rate of 80 μmoles/KG/days which is indicative that directelectron reduction is occurring [Rearden, E. J. “Zero Valent Iron stylesof corrosion and inorganic control of hydrogen pressure buildup” Env.Sci. Tech, 2005, 39 (3311-3317)]. However, we have found that theelimination and hydrogenation reductive pathways for the reduction ofTCE can be favored by increasing the number of catalytic sites on theZVI particles even at loading of 5.3% by weight. The 0.25% of graphitecarbon which remains in sponge iron from its manufacturing processapparently adds sufficient sites to the ZVI particle to favor theelimination and hydrogenation pathways.

The catalytic properties of the graphite carbon in sponge iron wereidentified in U.S. Pat. No. 5,975,798 as producing atomic hydrogen. Thiswas indicated from the absence of increases in pH that results fromhydroxyl formation that is produced from iron corrosion reactionsaccording to iron corrosion chemistry. Reduction reactions involvingatomic hydrogen results in the production of hydrogen ion, which canneutralize the hydroxyl ions, produced from the corrosion of iron.Increases in the number of catalytic sites on the ZVI can catalyze theelectrons to form atomic hydrogen and appears to limit the electronsinvolvement in direct reduction (hydrogenolysis). The results ofcorrosion studies employing sponge iron particles containing 0.25% byweight of graphite carbon shows no apparent rate of hydrogen gasproduction, which is generally associated with the corrosion of iron.The lack of hydrogen gas production is indicative that the electrons arebeing catalyzed to form atomic hydrogen.

We have observed that the 0.25% by weight of graphite carbon thatremains in sponge iron particles from the manufacturing process whenused at loadings of 5.3% by weight to treat TCE favors the eliminationand hydrogenation pathways. The results of analysis for the reactionproduct distributions show the cis DCE to be at non-detectable levels,the ethylene content is 16 μmole %, the ethane content is 75, mole % andother ethenes and ethanes amount to 9 μmole %. The first order rateconstant for the reduction of TCE was also observed to increase due tothe presence of the graphite carbon when compared to that determined forthe electrolytic type iron particles, which contained negligible amountsof carbon. The first order rate constant for the reduction of TCE usingthe sponge iron and electrolytic type iron was determined to be 0.066hr⁻¹ and 0.016 hr⁻¹, respectively.

Increased loadings of the sponge ZVI particles are also accompanied byan increase in ferrous ion production according to iron corrosionchemistry. This results in an increase of contaminants by reactions withferrous ions and oxyhydroxides resulting from the corrosion of iron inthe presence of ZVI and dissolved oxygen [Satapanajaru, T. et al “GreenRust and Iron Oxide Formation Influences Metolachlor DechlorinationDuring Zero Valent Iron Treatment” Env. Sci. Tech. 2003, 37(5219-5227)]. These insoluble ferrous oxyhydroxide compounds can alsocomplex with soluble reduced contaminants and remove them from solution.We have observed atomic hydrogen. However, by the addition of sufficientquantities of catalyst one can create sufficient numbers of catalyticsites on the ZVI surface to favor atomic hydrogen formation andreductions that primarily follow the elimination and hydrogenationpathways rather than the hydrogenolysis pathway.

In the preparation of our iron particles for the treatment of largevolumes of contaminated water for agriculture use and human activity,enriched graphite carbon was preferred over the use of metal catalystsuch as nickel. The release of carbon to the aqueous system beingtreated due to the corrosion of iron poses no toxic threat. The releaseof toxic catalytic metal ions during the corrosion of the iron cancontaminate the aqueous supply. The carbon is also less expensive thanthe metal catalyst and more resistant to poisoning by agents encounteredin the treatment of contaminated water. We have observed this to occurin the treatment of agriculture wastewater.

The treatment of agriculture waste water for removal of soluble seleniumVI for agriculture reuse employing sponge iron containing 0.25% carbonby weight at ZVI loadings of 0.5% by weight in presence of D.O. resultedin a 37% reduction of selenium VI directly to elemental selenium in 50hours. The use of atomized ZVI containing 2% by weight of nickelresulted in negligible removal of selenium VI over the same period oftime. The presence of poisoning agents in the agriculture wastewater isbelieved to be responsible for the inactivate nickel catalyst in theatomized iron particles.

Justification for Atomized Iron Particles Enriches with Graphite Carbonand Sulfur

The above results indicate that larger quantities of catalytic graphitecarbon are required in the ZVI particles to favor reductions of highlyreductive pollutants such as 1,1,1, TCA that follow the elimination andHydrogenation pathways rather than the hydrogenolysis pathways. However,the preparation of sponge ZVI particles with carbon contents above 0.6%by weight as identified in U.S. Pat. No. 5,975,798 is difficult. Thegraphite carbon exist as an alloy in the sponge ZVI particle andincreasing its amount would impact the optimum process conditions usedto reduce the iron ore to sponge ZVI. As a result, atomized ZVIparticles has been selected and enriched with over 4% by weight ofcarbon and 0.5% by weight of sulfur. These elements were added toincrease corrosivity (rate if corrosion) and reactivity (rate ofcontaminant reduction) of the iron for treatment of large volumes ofagriculture waste water, contaminated ground and surface water in aboveground conventional treatment systems and provide desired reductionpathways and products for contaminants that exhibit differentreductivities such as TCE and 1,1,1 TCA. The first order rate constantsfor the direct electron reduction of TCE and 1,1,1 TCA was reported tobe 3.9+3.6×10⁻⁴ L m⁻² h⁻¹ and 1.1×10⁻² L m⁻² h⁻¹ [Johnson et al“Kinetics of Halogenated Organic Compound by Iron Metal” Env. Sci. Tech,1996, 30 (2634-2640)].

The atomized iron is enriched with sulfur during the formation of theatomized iron particles. It is anticipated that the sulfur in theatomize iron will enhance the corrosion of iron to produce additionalelectrons and ferrous ions in the presence of dissolved oxygen. Thesulfur may also be reduced in the formation of the atomized iron to formsulfide which in the presence of ferrous ions forms ferrous sulfide onthe surface of the atomize iron particles. The combination of FerrousSulfide solutions added to ZVI particles has been reported to providegreater rates of reduction of contaminants than the use of ZVI particlesalone [Butler, E. et al “Factors Influencing Rates and ProductTransformations of TCE by FeS and Iron Metal” Env. Sci. Tech, 2001, 35(3884-3891)].

Evaluation of Enriched Atomized Iron

Bench scale kinetic studies were carried out in columns containing 25%by weight of the enriched atomized iron particles in 75% silica sand forthe reduction of TCE and VC. These studies also included the use of castiron particles containing 2.9% by weight of graphite carbon and spongeiron particles containing 0.25% by weight of graphite carbon to evaluatethe effect of increasing the graphite carbon on the reduction of TCE andVC. TCE and VC were selected because the TCE is reported to exhibit afirst order rate constant that is some 10 times faster than VC when thereduction pathways result from electron reduction (hydrogenolysis).

The results of these studies indicate that the first order rateconstants for the reduction of TCE and VC were determined to be 0.768hr⁻¹ and 4.44 hr⁻¹ using the enriched atomized iron particles, 0.345hr⁻¹ and 1.15 hr⁻¹ for the cast iron particles and 0.447 hr⁻¹ and 0.390hr⁻¹ for the sponge iron particles, respectively. These results showthat an increase in the quantity of catalytic graphite carbon in theiron particles favor atomic hydrogen reductions even at high loadings of25% by weight of atomized iron. The greater rate of reduction of VCcompared to TCE is indicative that the reductions involve atomichydrogen rather than direct electron reduction. At these loadings, 89%reduction of TCE and 100% reduction of VC were achieved in less than 3hours.

Results of pH

pH measurements within the test columns containing the enriches atomizediron at a dosage of 25% by weight with a 4% carbon content and 0.5% byweight sulfur content indicated the reduction of TCE and VC involvedatomic hydrogen. pH measurements collected within the column containingthe sponge iron at a dosage of 25% with a 0.25% by weight carbon contentindicated reductions of TCE and VC through direct electron reduction.

The results of pH measured in the test columns containing the enrichedatomized iron over a period of 30 hours show the initial pH to decreasefrom 7.5 to about 6.8 and remain constant over the duration of the pHmeasurements. These findings indicate the enrichment of the atomizediron was sufficient to catalyze the quantities of electrons produced atloadings of 25% by weight to form atomic hydrogen as was discussed inU.S. Pat. No. 5,975,798 with the use of sponge iron particles containingmuch lower weights of graphite carbon of less then 0.6% at ZVI loadings1.0%.

The result of pH measurements in the columns containing the sponge ironparticles show the pH increasing from 7.5 up to 9.0 within 2 hours andremaining constant at 9.5 over the test duration of 26 hours. Theseresults indicate that the reduction of the TCE and VC by the sponge ironis occurring by direct electron reduction at ZVI loadings of 25% byweight. As is expected, the quantities of graphite carbon present in thesponge iron does not provide sufficient sites on the sponge ironparticles to catalyze the quantities of electrons produced at theloadings to form atomic hydrogen.

SUMMARY

-   -   The quantity of graphite carbon and sulfur used to enrich the        atomized iron particles during their preparation should be at        least 4% by weight of graphic carbon and 0.5% by weight of        sulfur.    -   The function of the graphite carbon in the atomized iron is to        provide increased rates of degradation of contaminants and        provide additional sites on the surface of the iron particles,        which catalyze the electrons in the presence of water to form        atomic hydrogen. The reductions involving atomic hydrogen appear        to favor reactions that follow elimination and hydrogenation        pathways rather than direct electron reduction by hydrogenolysis        reaction pathways.    -   The function of sulfur is to increase the corrosivity of the        atomized iron particles to provide greater rates of production        of electrons and ferrous ions. The increased production of        ferrous ions can also contribute to greater rates of reduction        of contaminant by forming greater amounts of insoluble iron        oxyhydroxide flocs, formed during the corrosion of iron in the        presence of ZVI particles. The formation of these insoluble iron        oxyhydroxide flocs can also complex with soluble reduced        contaminants and remove them from solution.    -   The reactivity of the enriched atomized iron particles allows        the use of conventional above ground treatment systems for the        exsitu treatment of contaminated ground water and surface water        for agriculture reuse and human activity. The results of bench        scale column studies indicates that silica sand beds containing        25% by weight of enriched atomized iron particles have achieved        VC and TCE reductions of 100% and 89%, respectively at flow        velocities at 0.06 ft/hr in a bed depth of 0.17 ft. The        treatment systems employed would consist of industrial sized        large-scale reaction vessels capable of treating thousands of        gallons of water per day.    -   The enriched atomized zero valent iron in the form of an iron        wool could be used in smaller treatment vessels or cartridges        designed for treating smaller volumes of water for residential        or commercial use.    -   The increased reactivity of atomized iron wool enriched with        carbon and sulfur can provide treatment times of less then 3        hours and achieve desired inorganic and organic contaminant        reductions in small volumes of contaminated water at points of        residential and commercial use.

1. A material for use in a water decontamination process comprises zerovalent iron enriched with graphite carbon and sulfur.
 2. The material ofclaim 1 wherein said iron is in the form of particles.
 3. The materialof claim 1 wherein said iron is in the form of iron wool.
 4. Thematerial of claim 1 wherein said carbon graphite comprises at least 4%by weight and said sulfur comprises at least 0.5% by weight.